High‐ Q Tunable Waveguide Filter With Constant Absolute Bandwidth Using One Tuning Element Based on Modified Cavity Perturbation Theory

ABSTRACT This letter presents a high‐ Q tunable waveguide (WG) filter with constant absolute bandwidth (CABW). By adjusting the penetration depth of a single dielectric tuning element into the filter, frequency tuning is readily obtained without a complicated mechanism. Modified cavity perturbation theory (CPT) is adopted for the first time to deduce the quantitative relationship between the penetration depth and resonant frequency. At the same time, the tuning element also plays a vital role in